1. Field of the Invention
This invention relates to an improved system and method for shifting a discharge spot in an A.C. type discharge panel and, more specifically, to such system and method for improving the shift operation, including maximizing the shift operating margin.
2. Description of the Prior Art
Gas discharge panels known in the prior art as Self-Shift Plasma Display Panels, provide the function of shifting the discharge spot from a given cell to an adjacent cell. An example of such a panel is described in detail in U.S. Pat. No. 3,944,875 -- Owaki et al. assigned to a common assignee herein. Briefly, the self-shift plasma display panel described in this patent has an electrode configuration corresponding to that shown herein in FIG. 1.
In the prior art panel arrangement of FIG. 1, common electrodes y1 to y5, extending in a horizontal direction, are connected to a common bus yc in turn connected to a terminal Y. Although not illustrated in FIG. 1 to permit clarity of illustration of the electrode structure, the common electrodes y1-y5 are deposited on a substrate and have extending thereover a dielectric layer to insulate same from the discharge space. The panel further includes a second set of shift electrodes arranged in groups and illustrated in FIG. 1 by a1-d1 comprising a first group, a2-d2 comprising a second group, . . . and an-dn comprising an nth group. The corresponding electrodes a of each group are connected in common to a bus BA connected to a terminal A. Similarly, buses BB, BC, and BD are connected to the corresponding electrodes "b", "c", and "d", and in turn are connected to corresponding terminals B, C and D, respectively. As is apparent, the shift electrodes a, b, c, and d of all of the groups extend in generally parallel relation transversely to the common electrodes y and are formed on a second substrate as well including a dielectric layer overlying the electrodes and insulating same from the gas discharge space. The intersections of the shift electrodes and the common electrodes define discharge cells. Sequential application of shift voltages to the terminals A, B, C, and D causes shifting of a discharge spot from a given discharge cell in sequence to the respectively next adjacent cells as defined by the corresponding shift electrodes extending along the associated common electrode. Due to the respective, sequential application of the shift voltages to the terminals A to D, the shift voltages are frequently identified as being of phases A to D. Hence, the corresponding electrodes of each group receive the same phase shift voltage simultaneously.
The panel of FIG. 1 additionally includes a set of write electrodes w1 to w5 connected to corresponding terminals W1 to W5, respectively, which are disposed closely adjacent the initial shift electrode a1 of the first group of the set of shift electrodes. The write electrodes w1 to w5 respectively correspond to the common electrodes y1 to y5. The write electrodes would be provided on the same substrate and insulated from the gas discharge space by the same dielectric layer as that employed for the shift electrodes.
The panel, as is well known, is sealed and filled with an ionizable gas.
Accordingly, as above noted, each of the intersections of the write electrodes and the shift electrodes with the various common electrodes defines a discharge cell. Once a discharge, or discharge spot, is initiated in a given discharge cell, it can be shifted along the direction of the associated common electrode by the sequential application of shift pulses to the successive shift electrodes of each group, and for the successive groups of shift electrodes. Thus, the write electrodes are used to create initial discharges corresponding to information desired to be displayed and those discharges are shifted to desired positions in the panel or, in some cases continuously shifted throughout the length of the panel. New information may be written in for display as the previously written information is advanced, by shifting across the panel.
Where display of the information written into the panel is desired to be maintained at a given panel position, the shift voltage pulses may be supplied continuously to the shift electrodes at which the discharge spots displaying that information currently have been shifted, or in accordance with a further technique, a shift voltage may be applied alternately to two adjacent ones of the successive shift electrodes by application of alternate shift voltage pulses to the corresponding buses associated with those shift electrodes. In this regard, the shift voltage pulses may act as sustain voltage pulses to maintain the display.
The shifting of the discharge spot in a gas discharge panel, as is well known, is achieved by making use of the so-called priming effect. Specifically, when a discharge spot is generated at a given discharge cell, that discharge has a primary current, or priming effect, on the adjacent discharge cell due to the space charge created by the existing discharge, that space charge being formed of electrons, ions, and metastable atoms generated by the existing discharge at the given discharge cell. This space charge or priming effect serves to lower the firing voltage at an adjacent discharge cell below that level of firing voltage which otherwise would be necessary to create a discharge at the adjacent discharge cell. As a result, the lower limit, or minimum value, of the shift voltage required in a shift operation as above described is determined by this reduced value of firing voltage at the adjacent discharge cell resulting from the priming effect of the current discharge at the given cell. Conversely, this priming effect also sets an upper limit, or maximum value, of the permissible shift voltage pulse level, as determined by the firing voltage of a remote discharge cell receiving the same phase shift voltage pulse, and thus at the same time as the referenced adjacent cell intended to receive the discharge, by virtue of the common bus arrangement, to prevent misfiring, and to assure cancellation of the wall charge of the given discharge cell from which the discharge spot is shifted.
Specifically, with reference to FIG. 1, if the given cell illustrated at the circle P1 in FIG. 1 is currently undergoing a discharge and thus maintained in a "ON" condition, holding the discharge spot, the magnitude of the shift voltage pulse to be applied to the adjacent discharge cell defined by the circle P2 to which the discharge spot is to be shifted must be selected to be of a voltage level in accordance with the following conditions. First, the lower limit of that voltage level of the shift pulse must be greater than the firing voltage Vf1 at the discharge cell P2; as noted, the priming effect of the discharge cell P1 will cause a relative reduction in the normal firing voltage level required to create a discharge at the cell P2 in the absence of such priming discharge. Conversely, the shift pulse applied to cell P2 must be less than a level at which a discharge would be created at the cell P2' defined by the corresponding electrode b2 of the adjacent group (2) of shift electrodes. Specifically, the cells P2 and P2' simultaneously receive the same shift pulse phase through their respective shift electrodes b2 and b3 from the common bus BB. The level of the shift voltage which could create a discharge at cell P2' may be defined as Vf3.
Thus, under the condition of a given cell P1 undergoing discharge, the shift voltage for shifting that discharge to the adjacent cell P2 requires that the shift voltage be of a level exceeding the minimum firing voltage Vf1 at the adjacent cell P2 to which the discharge is to be shifted, but must be lower than the level Vf3 such as would create a discharge at the remote cell P2' which corresponds to the cell P2 but is in a different group of the shift electrodes. The difference of these firing voltage levels, (Vf3-Vf1) as defined by the adjacent, and the corresponding, remote discharge cells P2 and P2' (i.e., the "corresponding" cells being those energized from a common phase shift voltage over the same shift bus) defines the shift operating margin of the panel.
The shift operating margin of the panel thus is a critical condition in prior art shift-type plasma display panels, introducing corresponding difficulties in the manufacture and operation of such panels, including the driving circuitry therefor.